FIG. 6 shows suction and fuel feeding systems of a known four-cycle internal combustion engine. A cylinder 1, slidably fitted with a piston 2, is covered by a cylinder head 1a. A combustion chamber 1b is formed above the upper surface of piston 2. Head 1a is provided with a suction port 1c and exhaust port 1d periodically opened and closed respectively by a suction valve 3 and exhaust valve 4. Port 1d communicates with an exhaust pipe through an exhaust passage 1e. A muffler unit to silence the exhaust is interposed in the exhaust pipe.
Port 1c communicates with a suction passage 1f and is connected to an outlet part of a carburetor 5 which is a fuel feeding device. Carburetor 5 is provided with: a venturi 5a; a nozzle 8a in venturi 5a feeding fuel from a float chamber 8; a throttle valve 6 in the passage on the down-stream side of venturi 5a controlling the cross-sectional area of said passage to regulate the flow of a gaseous mixture; and a choke valve 7 in the passage up-stream of venturi 5a controlling the cross-sectional area of the passage to regulate the volume of air. An air cleaner is connected to the inlet of carburetor 5 to clean air fed to the inlet.
In the suction system including the fuel feeding device, when air containing fuel is sucked in, suction noises are generated. By silencing not only the exhaust noises, but also the suction noises, the noises of the engine as a whole can be controlled to obtain a quiet engine.
Suction noises have been neglected as compared with countermeasures against exhaust noises. Suction noises are generated by the following causes.
The first cause in suction sounds in the fuel sucking stroke and such fundamental suction sounds as the sounds of the momentary reverse currents of the exhaust pressure and compression pressure by the timing of opening and closing the suction valve and air currents.
The second cause is pipe resonance sounds generated when the suction passage influences the suction efficiency and engine operation. The suction sounds comprise 20 to 25% of suction noise, and the pipe resonance sounds comprise 75 to 80% of suction noise.
In an automobile, substantially all of not only the air cleaner and carburetor, but also the suction and fuel feeding systems, are housed in the engine compartment shielded with the hood.
In a motorcycle, not only the engine but also the suction system and fuel feeding system are not shielded. If many various devices are used to quiet suction noises, the suction system will become too large and will impair the appearance and design of a motorcycle.
There is required a means to efficiently silence suction noises, but which is small and light as to be able to be set within limited space without impairing the appearance and design of a motorcycle.